Adhering tiedown plies in composite construction

Abstract

We reduce core crush and ply wrinkling in composite honeycomb sandwich structure by preventing slipping of tiedown plies relative to the mandrel and to one another during autoclave curing. We produce superior panels with lighter weights, improved mechanical properties, and more predictable structural performance. The method involves applying a film adhesive to the tiedown plies in the margin of the part outside the net trim line. During heating of the autoclave and prior to the application of high pressure to the composite structure, the film adhesive cures to form a strong bond between the plys and to the mandrel. When pressure is applied, the tiedown plys are locked together and to the mandrel to prevent slippage between any layers in the panel.

We claim: 
     
       1. A method for adhering tiedown plies together in the manufacture of composite structure, comprising the steps of: (a) assembling on the layup mandrel a composite preform in the shape of the composite structure, the preform having at least one resin-impregnated laminate and at least two tiedown plies; and   (b) adhering the tiedown plies to one another with a film adhesive applied to the plies outside a net trim line of the composite structure wherein the film adhesive cures at a temperature lower than the resin in the laminate.     
     
     
       2. The method of claim 1 wherein the laminate includes bismaleimide matrix resin. 
     
     
       3. The method of claim 1 wherein the preform includes a barrier film made from a bondable grade, polyimide adjacent the laminate. 
     
     
       4. The method of claim 3 wherein the preform includes a honeycomb core and an adhesive between the barrier film and core. 
     
     
       5. The method of claim 4 wherein the preform includes a film adhesive layer between the barrier film and the laminate. 
     
     
       6. The method of claim 4 wherein the preform includes a supporting scrim between the barrier film adhesive and the core to prevent sagging of the barrier film into the core cells. 
     
     
       7. The method of claim 6 wherein the preform includes a tiedown ply in contact with the core between the adhesive and core. 
     
     
       8. A method for reducing core crush in a chamfered composite honeycomb sandwich panel having a resin-impregnated laminate adhered to a honeycomb core, the core having a chamfer, comprising the steps of: (a) contacting a tiedown ply with the honeycomb core of the panel in the region of the chamfer to prevent slippage between the core and the laminate;   (b) assembling tiedown plies over outer surfaces of the laminate; and   (c) adhering the tiedown plies together and to the layup mandrel with a lower temperature curing adhesive applied to the tiedown plies outside a net trim line of the panel, wherein the adhesive melts and cures prior to the application of autoclave pressure and prior to the melt and flow of the resin in the laminate.     
     
     
       9. The method of claim 8 wherein the laminate includes a barrier film to prevent resin flow from facesheets of the laminate to the core cells, and wherein one tiedown ply is between the barrier film and core.